Display arrangement for aircraftcoordinating systems



4 Sheets-Sheet l Feb. 2.0, 1951 J. o. MESA DISPLAY ARRANGEMENT FORAIRCRAFT-COORDINATING SYSTEMS Original Filed OCt. 9, 1946 Attorney Feb.20, 1951 Original Filed Oct. 9, 3:346

J. O. MESA DISPLAY ARQANGEMENT FOR AIRCRAFT-COORDINATING SYSTEMS VIEWINGPOSITION 4 Sheets-Sheet 2 INVENTOR. Joseph O. Meso Feb. 20, 1951 4Sheets-Sheet 3 Urignal Filed Oct. 9, 1946 lli Time

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Joseph O. Mesa i lr BY lll J. o. MESA 2,542,825

DISPLAY ARRANGEMENT FoP` AIRCRAFTcooRD1NAT1NG SYSTEMS 4 Sheets-Sheet 4 0S e M Attorney Feb. 20, 1951 Original Filed Oct. 9,

Patented Feb. zo, 1951 c DISPLAY ARRANGEMENT FOR AIRCRAFT- COORDINATINGSYSTEMS Joseph 0. Mesa, Richmond Hill, N. Y., as signor, by mesneassignments, to the United States of America'as represented by theSecretary of the Navy Original application October 9, 1946, Serial No.

702,328, now Patent No. 2,524,987, dated October 10, 1950. Divided andthis application January 27, 1949, Serial No. '13,154

2 Claims.

This invention relates to a display arrangement for anaircraft-coordinating system and particularly to a display arrangementfor such a system in which position information received from aircraftin flight is classified automatically to indicate which one of a numberof altitude strata is occupied by each of the aircraft from whichposition information is received.

As a result of greatly increased use of air transportation, it is now acommon occurrence for large numbers of high-speed aircraft bound indifferent directions to be flying simultaneously in the same area. Theresulting congestion of airborne traffic along the importanttransportation routes and particularly near airports makes increasinglyimportant the systematic coordination and control of this traffic, whichof course necessitates the eicient and continuous collection ofinformation regarding the positions of aircraft in flight. Not only mustsuch a traffic control system function when visual contact amongaircraft and between aircraft and the ground is impossible, but also itis highly advantageous that the system be fiexible enough to permit eachaircraft and each ground station to investigate on its own initiativethe traffic situation in its vicinity. Such a system is described in anapplication Serial No. 617,020, filed September 18, 1945, in the name ofKnox McIlwain and assigned to the same assignee as the presentinvention, now abandoned.

1n accordance with this system, the airborne or ground station desiringto investigate the traiic situation utilizes automatic radio equipmentto transmit interrogating signals to other stations within range.Responses are received indicative of the position of the replyingstation, and either the interrogating or the replying signals are codedautomatically to give the signals distinguishing characteristicscorresponding to one of a number of predetermined altitude strata. Theequipment for receiving either the interrogating or the replying signalscontains an automatic decoding arrangement for eliminating repliesexcept from those stations located in the altitude stratum of immediateinterest to the interrogating station.

Using this system the pilot of an aircraft in flight may determine thepositions of all other aircraft in his vicinity and in the same or aneighboring altitude stratum. Likewise a ground station may determinethe positions of all aircraft within range in any desired altitudestratum. These positions may be displayed, for example, on thefluorescent screen of a cathode-ray oscilloscope for viewing by thestation operator. To

facilitate the coordination of airborne traiiic, particularly in thevicinity of an airport, the ground station may use a plurality of codedtransmitters or a plurality of receivers with differently adjusteddecoding circuits to obtain simultaneously a plurality of displays, onefor each of a number of altitude strata. Alternatively, aircraft in theseveral strata may be interrogated by a single transmitter with the useof a rapidly recurring sequence of altitude codes, the responses foreach altitude stratum being displayed on a separate surface.

It thus has been proposed that a station for coordinating airborne trafcbe equipped with from two to ten or more display surfaces on which anoperator or operators may observe the location of aircraft in thevicinity of the station. In accordance with one such proposal thevarious display surfaces with their respective adjusting circuits areplaced alongside each other. so that a different operator may observeeach display, or so that one or more operators may pay divided attentionto a number of displays. Although display arrangements of this characteroften are satisfactory for a small aircraft-coordinating stationhandling relatively light trafc, onfor a very large station whichrequires at least several operators for trafic observation anddirection, an unaided control operator of a station of intermediateimportance may become overburdened. His duties may include observationand direction of aircraft taxying on the ground, as well as frequentradio-telephonic communication with aircraft in flight. Under theseconditions he may not have time to look from one to the other of anumber of entirely separate displays each limited to aircraft flyingwithin specified altitude levels.

Nevertheless, the advantages of displays which are readilydistinguishable as to altitude may warrant their use, since altitudeseparation of aircraft according to direction of flight or to landingpriority is a recognized principle of airtrafc control. Ofcourse,'various methods of coding to permit automatic classificationaccording to altitude may be used. Coding may be accomplished, forexample, by distinctive modulation of interrogating Wave signals of asingle carrier frequency, or by assigning dierent frequencies todifferent altitudes. In any case the positions of aircraft are presentedon a plurality of display surfaces corresponding to a plurality ofaltitude strata.

Accordingly, it is an object of the presentinvention to provide a newand improved display arrangement for an aircraft-coordinating sys- -temwhich substantially avoids one or more of the limitations of thedescribed prior arrangements.

It is also an object of the invention to provide, in anaircraft-coordinating system, a new and improved arrangement forsimultaneously displaying the positions of aircraft flying in variousaltitude strata. in which provisions are made to facilitatedistinguishing the altitude stratum occupied by each aircraft.

It is a further object of the invention to provide a new and improveddisplay arrangement for an aircraft-coordinating system in whichaltitude-classified position information received from aircraft inflight is displayed in perspective to permit easy identiiicatidnof thealtitude Wstratum occupied by each such aircraft.

In accordance with the invention. a display arrangement, for anaircraft-coordinating system in which position information received fromaircraft in flight is classified automatically to indicate which one ofa number of altitude strata each such aircraft occupies, comprises aplurality of display surfaces having substantial displacements betweenthe surfaces for eifectively displaying representations of the positionsof those of the aircraft occupying the respective ones of the altitudestrata. The display arrangement also comprises an optical system forviewing all of the display surfaces as if in one field but as separatedfrom each other axially along the optical system to permitidentification of the stratum associated with the representation of anyaircraft on any of the surfaces.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawings, Fig. 1 is a schematic representation of anaircraft-coordinating system, including a plurality of positionindicators, to which the present invention conveniently may be applied;Fig. 2 is a partial view of a control disc for use in the arrangement ofFig. 1; Fig. 3 is a graph of the signals present at related times invarious portions of the system of Fig. 1 during operation thereof; Fig.4 is a schematic representation of an alternative aircraft-coordinatingsystem, including a plurality of position indicators, to which thepresent invention suitably may be applied; Fig. 5 is a graph of thesignals present at related times in various portions of the system ofFig. 4 during operation thereof; and Fig. 6 is a perspective view of anarrangement, in accordance With the present invention, of the displaydevices included in the position indicators of the aircraft-coordinatingsystem of Fig. 1 or Fig. 4.

Referring to Fig. 1 of the drawings, there are illustrated schematicallyan interrogating station of an aircraft-coordinating system and arepresentative airborne replying station from which the interrogatingstation is receiving position information. The transmitter portion ofthe interrogating station comprises a lamp I for providing light adaptedto actuate a photocell bank II. Interposed between the lamp II) andphotocell bank Il is a signal-control disc I2 fastened to the rotatableshaft of a signal-control motor I3. A connection is made from photocellbank Il to a transmitter unit I4 including a generator of aradio-frequency carrier 4 signal and a modulator therefor, and theoutput circuit of unit I4 is connected to a horizontally directionalantenna l5 having a conventional rotating reector structure I6.

Within range of the interrogating transmitter is an airborne station 20having an omnidirectional receiving antenna 2|. The airborne stationfurther comprises a unitv22, including demodulator, decoder, and triggercircuits and having an input circuit coupled to antenna 2| and an outputcircuit coupled to an airborne reply transmitter 23. An omnidirectionaltransmitting antenna 24 is provided for the replying transmitter 23.

The interrogating station also includes an omnidirectional receivingantenna 25 and an amplifler-demodulator unit 26 having an input circuitcoupled to antenna 25 and output circuits coupled to each of veindicator devices 3I-35. The indicators 3|-35 correspond respectively tosuccessive altitude strata, identified by the numbers 1-5, andarenumbered accordingly. The mean heights of the altitude strata mayvary from low to high in the order named. YThe indicators are identicaland only indicator 3| for No. 1 stratum is shown in detail. The leadsfrom unit 26 to indicator 3| are connected to the input circuit of apulse amplifier 36 for No. 1 altitude stratum, the output circuit ofamplifier 36 being coupled to a beam-modulating electrode 31 and acathode electrode 38 of a cathode-ray device 35. Device 39 is one of aplurality of cathode-ray devices, each indicator having such a devicefor the corresponding altitude stratum. Rotatable about the neck ofcathode-ray device 39 is a yoke 40 containing a beam-deflectingarrangement whose terminals are brought to slip rings 4| and 42,contacted by brushes 43 and 44 respectively.

To provide for rotation of yoke 46 a gear 45 is aiixed to the peripheryof the yoke. A rotatable driving rod 50 passes in tangentialrelationship to gear 45 and to similar gears, not shown, in the otherfour indicators, any suitable mechanical coupling without reversal ofsense of rotation being supphed between the portions of the driving rod50 adjacent each such gear. Rod 50 carries a worm 5| meshing with thegear 45 and also carries similar worms 52-54, similarly operativelyplaced in the indicators 32-34 respectively. A worm 55, operativelyplaced in the No. 5 indicator 35, however, constitutes a screw ofopposite lead from that of worms 5l-54, causing rotation of the deectingyoke in indicator 35 in the opposite sense. Shaft 5D is connected bymeans of a bevel gear 56 to another shaft 51, which passes through anazimuth-control motor 58. Another bevel gear 59 on shaft 51 drives ashaft 6| fastened to the rotatable reflector I6. Reflector I6 isgrounded through a slip ring 62 on shaft 6| and the brush 63. ,The sevenbevel and worm gears have ratios such that one revolution of reflectorI6 occurs during one revolution of yoke 40 and also of each of the yokesin indicators 32-35.

The photocell bank I I houses seven phototubes, comprising apulse-generating phototube 69, sweep-signal-generating phototube 10, andve keying phototubes 1I-15. One element of each phototube is grounded.The other element of the pulse-generating phototube 69 is connected to aconventional pulse amplifier in transmitter unit I4 and the otherelement of the sweep-signalgenerating phototube 10 is connected to theinput circuit of a sweep-signal amplifier 16. The output signal-ofampliiler 18 is supplied to the rotatable beam-deilecting arrangement inindicator 3| through brush 43, and the other brush 44 is grounded tocomplete the circuit. The output signal of ampliiler 16 also is suppliedin the same way to the other indicators 32-35. Keying Dhototube 'II isconnected to a control circuit in the No. 1 pulse ampliiler 36. Likewisekeying Phototubes 12--15 are connected to similar control circuits inthe indicators 32--35 respectively.

Fig. 2 shows in greater detail part of the face of signal-control discI2 and also shows signal-V control motor I3. Photocell bank II as shownin Fig. 2 is a stationary compartmented housing in the front of which isa very narrow slit-shaped opening 18. Behind slit 18 are placed inseparate light-shielding compartments the seven phototubes 69-15arranged in the order of decreas- 111g radial distance in relation todisc I2. The disc has transparent openings located so that light passingthrough them and through slit 18 falls on one of the phototubes. Thus,at an extreme radius on disc I2 are narrow, regularly spaced groups ofslit-shaped openings 19 for registration with the compartment containingpulsegenerating phototube 69. Peripherally aligned with slots 19 are thenarrow ends of wedge-shaped openings 80 adapted to permit increasingamounts of light to fall on phototube during their passage in iront ofslit 18 and thus to generate sweep signals of saw-tooth amplitude. Atprogressively smaller radial distances from the center of disc I2 arelocated five keying openings 8I--85 for generating a succession ofkeying signals in phototubes 1I15, respectively. The quadrants of discI2 are identical and each contains ve pairs of coding slits 19, fivewedge-shaped openings 80,

and live keying openings Ill-85.

In the operation of the arrangement of Figs. 1 and 2, motor I3 causesdisc I2 to rotate in front of photocell bank II. Light from lamp I0passes through coding slits 19, as they come into register with slit 18,and excites phototube 69, which generates coded interrogating pulses forapplication to the pulse amplifier in transmitter unit I4. In themodulator of unit I4 the ampliiied pulses modulate the carrier signalgenerated in that unit. 'I'he modulated carrier signal is applied todirectional antenna I5, whereby the interrogating station transmitsdirectional, coded interrogating signals to aircraft in flight.

The interrogating signals are received at omnidirectional antenna 2l ofairborne installation 20. In the unit 22 the received signals aredemodulated and then decoded in a manner to be described moreparticularly hereinafter. 'Ihose signals which pass the decoding circuitcause a trigger circuit in unit 22 to develop a triggering pulse. Thispulse triggers the reply transmitter 23, thus causing a reply signal inthe form of a pulse of carrier-frequency energy to be applied to antenna24.

The reply signal is received at antenna 25 of the interrogating stationand applied to unit 26, where it is amplified, demodulated, and thensupplied to the input circuits of the pulse amplifiers in all iiveindicators 3 I-35, in one of which a display. indicative of thepositiorrof the replying aircraft, appears in a manner to be describedhereinafter. To prevent the receiving circuits of units 22 and 26 fromresponding to the same signals, it is advisable that transmitter unitsI4 and 23 gencrate carrier signals of diierent frequencies, although allinterrogating units and all replying cies the same as those oi units I4and 23 respectively.

Systems for coordinating the movements of aircraft in night almostinvariably involve division of the air space into a number of altitudestrata, each, for example. one thousand i'eet in height. To eiiectclassication of the position infomation received from aircraft in iiightthe interrogating signals may consist of a succession of pairs ofpulses, diilerent separations between the individual pulses of each pairbeing assigned ior the mean altitude o1' each o! a number of differentaltitude strata. This is accomplished by the arrangement of-the codingslits 19 in Fig. 2. As these slits pass in front of phototube 69 thespacing between the pulses of successive pairs of pulses increases. Apair of slits of maximum spacing in one quadrant of disc I2 has justpassed the phototube as illustrated in Fig. 2, while a pair of slitshaving minimum spacing in the next quadrant of the disc is approachingthe phototube. The resulting pulses are illustrated in the graph of Flg.3a, where the vertical coordinate represents the amplitude of thepulses. The coded pulses 19' illustrated are graduated from closelyspaced pairs corresponding to No. 1 altitude stratum to the'most widelyspaced pairs corresponding to No. 5 stratum. In the drawings therelative pulse spacing has been exaggerated for clarity of illustrationand the scale is too small to permit indication of the duration of anindividual pulse.

Aft r the carrier signals, pulse-modulated with the code spacingrepresenting the altitude strata, are radiated by antenna I5, receivedat antenna 2|, and demodulated in unit 22, the coded pulses are sent tothe airborne decoder circuit in the latter unit. In one form of decodercircuit the first of a pair of pulses is used to cause the generation ofa single pulse in a well-known keyed pulse-generation circuitarrangement which may be called a univibrator. 'I'he length of the pulsethus generated is determined by a circuit whose time constant dependsupon the position of a barometric device. Thus after an elapsed timedepending on the altitude of the aircraft carrying the decoder the pulseceases. This pulse may be applied to a conventional diierentiatingcircuit to obtain two impuls;s of opposite polarity, one a't the leadingand one at the trailing edge of the relatively long pulse generated bythe univibrator. 'Ihe latter impulse is used to place a triggeramplifier in an operative condition for a period of time starting afterthe elapsed time determinzd by the altitude of the aircraft and lastingVlong enough to accommodate the code spacings corresponding to the rangeof heights included in one altitude stratum. If the second pulse fromthe interrogating station arrives at the trigger amplier during thisperiod of time, it is amplified and applied to modulate the carriersignal generated in the reply transmitter 23. Suitable decoder circuitsare illustrated in Fig. 6 of the application Serial No. 617,020 referredto above; suitable univibrator and associated circuits are shown indetail in Fig. 4 of the same application.

Operation of the system of Figs. l and 2 conveniently may be describedfurther with reference to the graphs of Fig. 3, all of which are onrelated time scales, using for illustration two separate airborneinstallations 20, one in No. l stratum and rather near the interrogatingstation, and

units in the system should use carrier frequenu the other in No. 3stratum and relatively vdistant 7 therefrom. Passage of wedge-shapedopenings 8l in front of phototube 10 results in the' generation of thesweep signals 80' shown in Fig. 3b. Passage of keying openings 8|-85 infront of phototubes II-I respectively causes the successive generationof keying signals 8|-85' respectively, illustrated in Figs. 3c-3grespectively. The sweep signals are initiated with the transmission ofthe second pulse of each pair, and the keying signals 8|'-85' are eachcoincident with the sweep signal following the pulses coded for strataNos. 1-5 respectively, The coded pulses 'I9' Aof Fig. 3a are received inthe installation 2|! in the nearer aircraft after a short time interval,as indicated in Fig.. 3h. The decoder circuit in that aircraft isconditioned automatically by an as'- sociated barom-tric device toamplify only the second pulses of only those pairs of pulses having theshort spacing corresponding to No. 1 stratum. After receipt of thissecond pulse, however, the trigger circuit causes the reply transmitter23 in that aircraft to transmit a single pulse, as illustrated at 9| inFig. 3i. As .shown in Fig. 3k, the same interrogating pulses arereceived shortly thereafter by the installation in the aircraft in No. 3stratum, which, however, responds only to those pairs of pulses codedfor No. 3 stratum, transmitting reply pulses one of which is illustratedat 93 in Fig. 3m. Fig. 3u illustrates the reply pulses as receive-d atantenna of the interrogating station, pulse 9 I being received from theaircraft in No. 1 stratum and pulse 53' from the aircraft in No. 3stratum.

The keying signals 8|-85' serve to condition for operation the pulseamplifiers of the five indicators in succession. Thus when the replyingpulses 9|' reach the interrogating station only the No. 1 pulseamplifier 36 is operative due to keying signal 8|', while only No. 3indicator is operative due to keying signal 83 wh-n the pulses 93 arrivefrom the aircraft in No. 3 stratum. Cathode-ray device 39 in indicator3| and the similar devices in the indicators 32-35 preferably areadjusted so that, when no sweep signal is applied to their deectingyokes. the cathoderay b ams are focused on datum points locatedapproximately at the centers of the fluorescent screens or other displayarrangements included in the devices. A sweep signal then causesdeflections radially outward from these centers. The beams are preventedfrom producing a visual signal, however, except when the appropriatekeying signal permits modulation of the beam.

When the sweep signal 80' is applied to the detlecting yokes, the beamsare deflected toward the peripheries of the screens in a directioncorresponding to the positions of the deflecting yokes. Since the yokesrotate in synchronism with antenna reflector I6, they may be adjusted sothat the azimuthal relation to the interrogating station of the portionsof the sky covered by antenna I5 is indicated on each indicator. Forexample, when the reflector I6 faces north sweepsignal deflections arein an upward direction, while when the reflector I3 has rotated 90degrees and faces east sweep-signal deflections are in a horizontalangular-direction rotated 90 degrees with reference to theupwarddirection. In this way the azimuthal information conveyed by the angularposition of the beam is received from aircraft which reply only whenreector I6 permits transmission of interrogating signals to aircraftlocated in the corresponding direction. The reason for the oppositesenses of rotation of the Nos. 1-4indlcators will be pointed outhereinbelow.

The method of indicating range information also is illustrated in Fig.3. During the propagation of interrogating pulses 1S and replying pulses9| and S3 deflecting signals ll' of increasing amplitude are applied tothe deflecting yokes in the five indicators 3|-35. The instantaneousvalue of one of these sweep signals is proportional to the time elapsedsince the transmission of the corresponding interrogating pulse andhence to the range from the interrogating station of the replyingaircraft. Thus, as may be seen by comparing Figs. 3b and 3u. thereplying signals 8| modulate the beam and so appear on the screen of No.1 indicator 3| when the beam has been deiiected a relatively smalldistance from the center of the screen, while the replying signals S3'appear on the screen of No. 3 indicator 33 after a much greater deectionof the beam corresponding to the greater distance of the aircraft in No.3 stratum. Hence representations are formed on the screen of eachindicator having, with reference to a datum point centrally located oneach screen, angular positions and radial distances correspondingrespectively to the azimuth and to the range information received fromthe aircraft.

If desired, rotation of the direction of sweeping about a datum point oneach cathode-ray screen may be obtained by electronic means, using thesystem of radial scanning synchronous with an effectively rotatingantenna, disclosed in an application Serial No. 433,173, filed March 3,1942, now abandoned, in the name of Harold A. Wheeler and assigned tothe same assignee as the present invention. Moreover, all of the signalsillustrated in Figs. 3er-3g also may be generated by electronic means,using the coding circuit shown in Fig. 4 of the application Serial No.617,020 referred to above, well-known pulse-forming and sweep-generatingcircuits, and conventional counting and keying circuits to provide thesuccession of keying signals illustrated in Figs. 3c-3g. The arrangementof the present invention may be applied to a system providing aplurality of position indicators of the type described regardless of thecircuits used to procure the indications.

In Fig. 4 there is illustrated schematically a. system similar to thatshown in Fig. l, but in which the coding is accomplished in the airborneinstallations and decoding is done in the receiver circuits of theinterrogating station. Referring to Fig. 4, a pulse and sweep-signalgenerator consists of a conventional generator of a series of shortpulses having substantially rectangular Wave shape and a keyedsweep-signal generator of well-known design for generating signals ofsaw-tooth wave shape starting coincidently with each of the pulses.Generator is coupled to an R. F. generator and modulator H4, the outputcircuit of which is coupled to a directional antenna M5 having arotatable reflector H6. Units and ||4 and antenna ||5 constitute theinterrogating portion of an interrogating station which is within rangeof at last one airborne installation |20.

The airborne installation |20 includes an omnidirectional receivingantenna |2| connected to a unit |22 including demodulator, trigger, andcoder circuits. The unit |22 has an output circuit coupled to anairborne reply transmitter |23. An omnidirectional antenna |24 iscoupled to the output circuit of the reply transmitter |23.

yoke in No. 5 indicator Il and the yokes in the 'l'. At theinterrograting station an omnidirectional receiving antenna is connectedto the input circuit of an amplifier-demo'dulator unit |26, the outputcircuit of which, in turn, is coupled to an indicator |3| for No. 1altitude stratum and to similar indicators |32-I35 for strata Nos. 2-5respectively. The indicator |3| includes No. 1 decoder and triggercircuits |36, a No. 1 pulse amplifier |38,.and a control element |31 ofa No. 1 cathode-ray device |39. The output circuits of demodulator |26are coupled to units |36 and |38 arranged in tandem and thence to thecontrol element |31. Similar arrangements are provided in each of thefive indicators. The coder circuit of unit |22 and the decoder circuitsin each of the five indicators may take the forms shown in'Figs. 4 and 6respectively of the application Serial No. 617,020 referred to above.

The cathode-ray device |39 has a rotatable beam-deflecting arrangement|40 having slip rings |4| and |42 and a peripheral gear |45 affixedthereto. A drive shaft |50 passes in tangential relation to the gear |45and to similar gears in each of the other indicators and carries wormsillustrated by worm |5| meshing with its gear |45. As in the system ofFig. 1 the worm,

not shown, associated with the No. 5 indicator has a reversed lead, andthe drive shaft |50 is coupled mechanically by means of bevel gears |56and |59 to the reflector ||6 and to a driving motor |58. The slip ring|42 on deilecting yoke is grounded, as is a similar slip ring |62 on theshaft supporting reflector H6. The remaining slip ring |4| on deectingyoke |40 in indicator |2| and similar slip rings on the yokes in theother indicators are connected to the sweep-signal output circuit ofpulse and sweepsignal generator The operation of the arrangement of Fig.4 will be described in connection with the graphs of Fig. 5, al1 ofwhich are on related time scales. The pulse output of pulse andsweep-signal generator is illustrated in Fig. 5a, the vertical height ofthe pulses indicating their amplitude. The pulses are generated at aregular rate which may correspond to the rate at which the pairs ofpulses shown in Fig. 3a are generated. However, the pulses generated byunit are uncoded. Simultaneously with the generation of each pulse asweep signal of the form indicated 'l In Fig. 5b starts to appear at theproper terminal of unit This sweep signal is applied to thebeam-deflecting yokes in all five indicator units |3|-|35. The pulsesappearing at the proper terminal of unit are used to modulate theradio-frequency carrier signals generated in unit ||4, and thepulse-modulated carrier is radiated by antenna ||5 in the directiondetermined by reflector 6 to any airborne installations |20 within rangeof the interrogating station.

Operation of the arrangement of Fig. 4 will be described using asexamples the signals that would be present if one airborne installationwith a rather close range is present in No. 1 stratum and anotherairborne installation of relatively great range is present in No. 3stratum. Considering first the installation |20 in No. 1 stratum.`

the uncoded interrogating signals are received at its antenna |2| aftera time delay dependent upon the distance between the interrogatingstation and the airborne unit, as illustrated in Fig. 5c. These signalsare demodulated in unit |22 and applied to trigger a coder circuit inthe same unit. Immediately after receipt of each interrogating pulse thecoder circuit generates a pair of pulses having a spacing correspondingto the altitude stratum of the aircraft, in this case, the short spacingassigned to No. 1 stratum. These paired pulses, which are illustrated inFig. 5d, are applied to modulate the carrier signal generated byairborne reply transmitter |23, and the modulated signals are radiatedby antenna |24. In an analogous manner the interrogating signals arereceived at a still later time by the installation |20 in the moredistant aircraft in No. 3 stratum, the received signals beingillustrated in Fig. 5e. In this case, however, the reply pulses arespaced more widely in accordance with the code established for No. 3altitude stratum, this spacing being determined automatically bybarometric equipment in the aircraft. The replying pulses from theaircraft in No. 3 stratum are illustrated in Fig. 5f.

After additional elapsed time, corresponding to the distances betweeneach of the two aircraft involved and the interrogating station, thecoded reply pulses are received at antenna |25 of the interrogatingstation. For each interrogating pulse one pair of coded pulses isreceived from each aircraft in the portion of the sky then covered bydirectional antenna ||5, as shown in Fig. 5g. The No. 1 decoder in unit|36 of indicator |3| eliminates all signals except those coded for theNo. 1 altitude stratum. The second pulse of each pair of properly codedpulses, however, is applied to a trigger circuit in unit |36 which formsthe series of pulses illustrated in Fig. 5h. The latter pulses areapplied to No. 1 pulse amplifier |38 and thence to the cathode-raybeammodulating electrode |31 of the cathode-ray device |39. Each one ofthe five indicators is equipped with a decoder circuit to select pairedpulses having the spacing corresponding to its respective altitudestratum. Thus decoder and trigger circuits in the No. 3 indicator |33produce pulses illustrated in Fig. 5j :ln response to the coded pulsesreceived from the aircraft in No. 3 stratum. The pulses formed by thesetrigger circuits are amplified in a corresponding pulse amplifier andapplied to the control electrode of a corresponding cathode-ray devicein No. 3 indicator. Accordingly representations similar to thoseobtained in the system of Figs. 1-3 are formed on the screens of the veindicators |3|-|35.

The arrangements of Figs. 1 and 4 each have advantages, whicharrangement is used Vbeing a matter of standardization as well as oftechnical preference. For each interrogating signal in the arrangementof Fig. 4 reply signals are received from all strata. To preventinterference between circuits carrying interrogating signals andcircuits carrying replying signals it is advisable that the carriersignal generated by interrogating unit ||4 be of slightly differentfrequency from that of the carrier signal generated by reply unit |23.

Both Fig. 1 and Fig. 4 show an interrogating station of anaircraft-coordinating system in which the station transmits directionalinterrogating signals to aircraft in flight. The station receives inresponse to those signals from such aircraft within range of the stationposition information, specifically, information on azimuth and rangewith reference to the interrogating station, this information beingclassified automatically to indicate which one of a number of altitudestrata each of the replying aircraft occupies. The systems of Fig.' 1and Fig. 4 both include a plurality of cathode-ray devices having aplurality of fluorescent screens included in the devices. Each screen issupplied with position information which is automatically limited to onestratum of the sky surrounding the station and is displayed in the formof a map or plan view indicating azimuthal relation and the range ofaircraft in the vicinity, so that the screens respectively displayrepresentations of the positions of those of such aircraft occupying therespective ones of the altitude strata. v

Fig. 6 shows a display arrangement, according to the present invention,of a plurality of display surfaces such as those associated with theindicators of Fig. l or Fig. 4, each of which includes a cathode-raydevice. Thus cathode-ray tubes |1|, |12, |18, |14, and |15 are includedin Fig. 6, and each tube has a display surface on which may be presenteda plan position indication of the air craft within range of theinterrogating station in altitude strata Nos. 1, 2. 3, 4, and 5respectively. In a particularly advantageous form of position-indicatingarrangement for use in the present invention the display surfaces arefluorescent screens capable of producing indications of high brillianceand long persistence, so that the indications produced for each positionof the directional antenna remain visible until the rotating antennareturns to that position. The display surfaces have substantial spatialdisplacements between each other.

The remainder of Fl'g. 6 illustrates an optical system for viewing allof the display surfaces as if in one field. The optical system includesan optical axis |10, passing through a viewing position |80 and thedisplay surface of tube |15. For convenience of illustration the opticalaxis |10 is considered to lie in a horizontal plane. Of course, thisoptical axis may be subjected to changes in direction at any pointstherealong merely by placing totally reflecting mirrors on the axis,permitting convenient placement of the cathode-ray device with respectto viewingposition |80. 'I'he remainder of the display surfaces arepositioned off the optical axis 10, the displacement between the displaysurfaces thus preventing the other display surfaces and associatedeouipment from obscuring the surface of tube 15. The optical system alsoincludes an individual partially reecting mirror intersecting opticalaxis |10 for each of the display surfaces positioned oif the opticalaxis. Thus, partially reflecting mirrors |16, |11, |18, and |19 areprovided for the display surfaces of cathode-ray tubes |1|, |12, |13,and |14 respectively.

The partially reflecting mirrors shown in Fig. 6 are arranged invertical planes making 45-degree angles with the vertical plane throughoptical axis |10. Hence the auxiliary optical axes connecting each ofthese display surfaces with the principal optical axis |10 are at rightangles to the axis |10 and in the horizontal plane including axis |10.However, it will be understood that each of the partially reflectingmirrors may have any of numerous positions, provided that the mirrorintersects the axis 10 and that the angle of reflection made with axis|10 equals the angle of incidence between the partially reflectingmirror and the auxiliary optical path to the respective display surface.Each of the display surfaces may be displaced from the horizontal planeincluding axis |10, provided that the surface faces the axis and thatits respective partially reflecting mirror is tilted correspondinglyfrom a vertical plane.

As shown in Fig. 6, the optical distance from viewing position |80 tomirror |16 and thence to smaller than the optical distance from miticato mirror |11 and thence to the display surface of tube |12. The latterdistance in turn is smaller than the length of the optical path fromposition |80 to the tube |13, and the distances to tubes |14 and |15likewise are progressively greater. Thus the effective optical distancesin the optical system between viewing position |80 and each of thedisplay surfaces are different, and differ sequentially from displaytubes |1| to |15. Since these tubes correspond to the first to fifthaltitude strata in the order mentioned, the effective optical distancesdiffer sequentially in the altitude sequence of the strata correspondingto the display surfaces.

In the operation of the arrangement of Fig. 6, light from tube |15passes through each of the mirrors |19, |18, |11, and |16 to the viewingposition |80. There is some attenuation of this light due to reflectionfrom the back surfaces of each of the mirrors, so that it is preferablefor the display appearing on tube.|15 to be adjusted for highbrilliance. Part of the light from tube |14 is reflected from the frontsurface of mirror |19 and then passes along optical axis |10 through theremaining mirrors to the viewing position. Likewise, light from tubes|13 and |12 is reflected by the mirrors |18 and |11 respectively andpasses through the optical system in the same way to the viewingposition. Finally light from tube |1|, after partial reflection atmirror |16, passes directly to position |80 without further attenuation.Thus, the optical system described `permits viewing by an observer atposition |80 of all five of the display surfaces as if in one field ofview. Due. however, to the difference in length of the effective opticalpaths from the viewing position to each of the display surfaces, thesurfaces are viewed as separated from each other axially along theoptical system, permitting identification of the stratum associated withthe representation of any aircraft on any of the surfaces.

Operation of the arrangement of Fig. 6 may be illustrated by using asexamples displays of the positions of the two aircraft assumed to bewithin range of the interrogating station in the discussions of Figs. 1and 4. These aircraft are assumed to be in a generallynorth-northeasterly direction from the interrogating station, andindications of their positions are obtained when the directional antennaof the interrogating station is facing in that direction, at which timethe direction of the sweep signal applied to all the display surfaces isabout 25 degrees clockwise from the vertical in the upper right-handquadrant of the circular display surfaces as viewed from position |80.The direction of sweeping of the unmodulated beam over the displaysurface of the No. 5 tube 15 is shown by dotted line |85. If duringcontinued operation the directional antenna of the interrogating stationscans the sky from north through east, south, west, and back to north inthe order named, the direction ofv individual sweeps of the beam in tube|15 changes in a clockwise order, passing in turn through the upper andlower right-hand quadrants and the lower and upper left-hand quadrantsof the display surface. As pointed out hereinabove the deflecting yokeson the tubes |1||14 for strata Nos. 1-4 rotate in the opposite sense,vthat is, counter-clockwise, from the sense of rotation of the yoke ontube |15. During initial adjustments of the system the yokes on all ofthe tubes are amasar rotated to an adjustment in which their beams sweepupward when the directional antenna faces north. Thus inthe presentspecific example the beams in tubes Ill-|14 sweep over paths in theupper left-hand quadrants of the display surfaces in a direction about25 degrees counter-clockwise from the vertical. Upon reection at theirrespective mirrors |16|18, however, the sense of rotation of thedirection of sweeping, starting from the upward direction, appears tothe observer to be reversed. Thus the direction of the sweep signal inthe example appears to be about 25 degrees clockwise from the upwarddirection in all iive oi the tubes.

In general, each time areilectionfoccurs the direction of sweeping mustbe reversed. For example, if a totally reecting mirror were placed onthe optical axis near tube |15 to reflect the image of the tube, thenthat tube would require deilecting-yoke gearing having the same sensethird altitude strata., cause representations to anpear on the-surfacesof tubes |1| and |13, rspec-eV tively. The signals received from theaircraft in the No. 1 strata are illustrated in Fig. 311. at 9|' or inFig. 5h. This aircraft is relatively near the interrogating station andso dispatches to the station a reply signal which arrives there beforethe sweep signal of Fig. 3b or Fig. 5b hasattalned an amplitudesufficient to cause a large deflection of the beam from the centraldatum point on the screen of tube |1|. The display surface of tube I1|is hidden in the view of Fig. 6. but its boundary and horizontal andvertical lines through its central datum point are indicated in dottedlines. 'I'he resulting indication is shown on the screen of tube |1| at|8I. The reply signal from the aircraft in No. 3 stratum arrives at alater time with respect to the sweep signal. as illustrated in Fig. 3uat 93' or in Fig. 5i, when the sweep signal has attained an amplitudesuiilcient to reflect the cathode-ray beam almost to the periphery ofthe tube. This reply signal arrives when the indicator associated withtube |13 is keyed on, using the system of Fig. l. or arrives with a codewhich is acceptable only to the indicator circuits associated with tube|13, using the system of Fig. 4. Hence the resulting indication appearson the screen of tube |13 at |83. `As seen from viewing position |80,the indications |8| and |83 both appear-tto be in the upper righthandquadrant of the eld at about 25 degrees from the vertical, thusindicating the same azimuthal relation for the two indications, but withindication |83 much nearer the edge of the field.

It is preferable to align the display surfaces in the optical system sothat the centrally located datum points on each surface aresubstantially in register as viewed from position |88. When this isdone, it readily is apparent to the observer that indication |83represents an aircraft more distant from the station than the aircraftrepresented by indication |8I, but that the two aircraft have the sameazimuthal relation to the interrogating station. Furthermore, theobserver sees the indication |83 as if at a greater distance from hiseyes than the indication |8|,

thus permitting the two indications to be distinguished easily asregards the altitude strata occupied by the respective aircraft. Ofcourse. the sequence of distances may be varied at will by lnterchangingthe display surfaces or by varying the lengths of the various opticalpaths in the system. For example, the surface representing the higheststratum may be made to appear nearest to the observer. When there arenumerous aircraft within range occupying various altitude strata, theapparent depth separation of the position indications for the variousstrata very materially aids the observer in determining the distributionof airborne traihc in the area. If desired, the display screensthemselves may be given distinctive` characteristics, for example,distinctive colors, to permit easy recognition of the surface upon whichthe observer hasifocused hisattention. l

Many modifications of the arrangement of Fig. 6 will occur to thedesigner confronted with a particular display problem. The proportionsof the lengths of the various optical paths to the display surfaces maybe varied to enhance Vthe apparent stereoscopic separation of thesurfaces to any desired degree. The observer viewing the arrangement ofFig. 6 sees the tube |15 as the most distant and consequently thesmallest display surface. Nevertheless, the representation of aircraftvertically above each other in different strata may be made to appear tothe observer as aligned by attenuating slightly the sweepsignal voltagesin the nearer tubes in proportion to the nearness of the tubes. It alsomay be pointed out that the range indicated by representations of thetype described is a so-called slant range, meaning the length of thepath from the interrogating station to the aircraft rather than theprojection of this path on a horizontal surface. During usual operationshorizontal distances from the interrogating station are so much greaterthan the maximum altitude involved that this discrepancy in the rangeindication is quite negligible. However, if desired, the

' start of the sweep signal applied to a tube representing a higheraltitude stratum may be delayed by a time equal to that necessary toreceive a reply from the nearest aircraft in that stratum, that is, onedirectly overhead. If this is not done. there is a. small circular areaaround the datum point of the corresponding display surface which nevershows a representation of an aircraft.

While. there have been described what are at present considered to bethe preferred embodi-,t ments of this invention, it will be obvious tothose" skilled in the art that various changes and modi-l iications maybe made therein without departing from the invention, and it is,therefore, aimed in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

This application is a division of patent application, Serial No.702,328, led October 9, 1946, now Patent No. 2,524,987, issued October10, 1950.

What is claimed is:

1. In a wave-energy displaysystem for indicating the locations relativeto a ground station of aircraft operating in a particular altitude zone,a rst pulse transmitter operating on a first frequency, a directionalantenna rotatable about a vertical axis connected to said pulsetransmitter, pulse modulation means connected with said pulsetransmitter to control the energization thereof, a ilrst pulse receiverresponsive to signals of a second frequency located adjacent saidtransmitter, coding means connected to said pulse modulator te energizetransmit* r tc radi= ate pulsed signals corresponding to a particularaltitude zone and in a direction determined by the direction cf saiddirectional antenna, a second pulse receiver carried by said aircraftresponsive to pulse signals from said transmitter, an adjustabledecoding means connected with said receiver, altitude responsive meansconnected with said decoding means to render said decoding meansresponsive to signals corresponding to the altitude zone occupied bysaid aircraft, a second pulse transmitter operating on said secondfrequency carried by said aircraft and connected to said decoding meansto radiate a signal in response to coded signals corresponding to thealtitude of said plane, and indicator means responsive to the timeinterval between the transmitted coded signal from said rst pulsetransmitter and the received response at said iirst re-V ceiver toindicate the range to said aircraft and to the heading of saiddirectional antenna to indicate the direction of said aircraft from saidground station.

2. In an aircraft control system, a ground control station comprising afirst pulse transmitter operating on a first predetermined frequency andhaving a directional antenna rotatable in azimuth, means for energizingsaid rst transmitter to radiate a series of pairs of pulses ofpredetermined magnitude and of a spacing between said pairs of pulsesrepresenting a predetermined altitude zone, a mobile transponder carriedby 16 each of said aircraft comprising a first pulse receiver responsiveto radiated pairs of pulses from said first pulse transmitter, anadjustable decoder adapted to produce an operating pulse in response tothe application thereto of pairs of pulses of a spacing corresponding tothe adjustment of said decoder connected to said receiver,altitude-responsive means connected to said decoder to vary itsadjustment in accordance with the altitude of the aircraft, a secondpulse transmitter responsive to operating pulses from said decodingmeans to radiate a signal, a second pulse receiver at said groundstation responsive to signals radiated by said second pulse transmitter,and indicator means jointly responsive to the heading of saiddirectional antenna to indicate the direction of said aircraft from saidground station and to the time interval between the transmission of apair of pulses by said first transmitter andthe reception of a pulsefrom said second transmitter by said second pulse receiver to indicatethe range to said aircraft.

JOSEPH 0. MESA.

REFERENCES CITED The following references are of record in the ile ofthis patent:

UNITED STATES PATENTS Number Name Date 2,468,045 Deioraine Apr. 26, 19492,480,123 Deioraine vAug. 30, 1949 2,483,097 McIlwain Sept. 27, 1949

